The disclosure generally relates to liquid delivery apparatus and, more specifically, to implantable drug delivery devices. Implantable drug delivery devices are medical devices used to administer an infusate (e.g., medications/drugs, chemicals, solutions) to a predefined location in a patient (e.g., intrathecal delivery). The devices can be used to treat chronic pain, spasticity, or other medical conditions that would benefit from controlled administration of an infusate.
Chronic pain is a persistent condition in which the source of the pain cannot be treated. It afflicts an estimated 100 million people in the United States with annual costs exceeding $100 billion. W. A. Visser, “Combined spinal epidural anaesthesia,” Anaesthesia, vol. 54, p. 300, March 1999. It is often a symptom of incurable or intractable conditions like cancer, but it can also arise from severe trauma, limb loss, or other combat-related injuries. P. S. Tumber et al., “The control of severe cancer pain by continuous intrathecal infusion and patient controlled intrathecal analgesia with morphine, bupivacaine and clonidine,” Pain, vol. 78, pp. 217-220, December 1998; M. Carmichael, “The changing science of pain,” Newsweek, vol. 149, pp. 40-47, Jun. 4, 2007; T. J. Smith et al., “Pain management, including intrathecal pumps,” Curr. Pain Headache Rep., vol. 9, pp. 243-248, August 2005. If the chronic pain condition is severe, it may be treated with surgery, spinal cord stimulation, or the implantation of an intrathecal drug pump.
The technological aspects of intrathecal drug pumps have remained relatively unchanged for over a decade. Conventional intrathecal pumps consist of a battery-powered, programmable pump which is connected to or incorporates an infusate reservoir. The pump is implanted under tissue in the patient's abdomen and connected to a catheter that continues to a infusion/drug delivery point (e.g., a spinal entry point). K. Knight, “Implantable Intrathecal Pumps for Chronic Pain: Highlights and Updates,” Croat. Med. J., vol. 48, pp. 22-34, 2007.
In some intrathecal pumps, an electrically powered mechanism pumps the infusate from the reservoir to the infusion point. Such pumps can be used to control the dosage of the infusate and variable dosing protocols can be followed.
In other intrathecal pumps, the infusate is driven to the infusion point by a propellant exerting a positive and constant pressure on the infusate reservoir. Such devices require large rigid housings to contain both the infusate reservoir and the propellant. While such gas-driven pumps can be cost-effective, they suffer from infusate delivery problems when environmental factors change the pressure that the propellant exerts on the infusate reservoir.
The problem of variable flow in gas-driven pumps has been addressed using drive-spring diaphragms, for example as disclosed in U.S. Pat. No. 6,666,845. In such systems, a drive spring provides the constant pressure to collapse the infusate reservoir. Similar to the gas-driven pumps, the spring-driven pumps require a large and rigid housing.
A limitation of commercially available intrathecal pumps is that they provide only a single infusate reservoir, while preferred methods for pain management specify the administration of multiple medications with different dosing and delivery protocols. However, such single-reservoir systems require drugs to be mixed in a fixed ratio and dispensed from a single reservoir, which can be problematic with drugs that are incompatible and/or unstable when stored in a mixture for an extended period. Moreover, single-reservoir systems cannot independently control the relative flow rates of individual drugs.
The large, rigid nature of conventional intrathecal pumps limits the applicability of the technology. The utility of an intrathecal device can be represented by its volume efficiency. The volume efficiency is the ratio of the volume of the infusate reservoir when full to the volume of the entire intrathecal pump device. As the volume efficiency is limited by the architecture of the pump and its component sizes, improvements in volume efficiency in conventional intrathecal pumps is limited. A low volume efficiency can restrict the use of intrathecal pumps, for example preventing their use in pediatric applications, which require a significantly smaller pump than that which can be used in an adult.
Thus, there is need for an intrathecal drug delivery device that has a high volume efficiency, is volume-scalable, and can independently deliver multiple drugs.